This invention relates to interferometry, and more particularly to a speckle interferometry apparatus and method that utilizes a scattering reference plate that can incorporate phase shifting.
Optical interferometers are known which make use of the interference phenomena known as the "speckle effect," the speckled pattern seen when laser light is used to illuminate a rough surface. This invention utilizes the speckle effect, but it offers significant cost and performance improvements over conventional apparatus and methods.
U.S. Pat. No. 4,850,693 teaches a compact and portable moire interferometer for determining surface deformations of an object; and U.S. Pat. No. 4,794,550 teaches a method of extending the measurement range of the moire reference beam techniques by constraining the reconstruction of a surface contour based on a prior knowledge about the surface. These moire methods require that some form of a grating be created or projected onto the surface of the specimen, perhaps by the use of coherent laser light.
The physics of this invention are distinctly different from moire techniques. This invention measures deformations, displacements, and strains of an object, but it does not employ the "moire effect," in that no grating is created on the specimen or in the optical system. Only the "speckle effect" is used.
Speckle interferometry is known for use in measuring strain in structural members and mechanical components. U.S. Pat. No. 4,591,996 teaches a method and apparatus for measuring strain in structural members utilizing a laser beam to illuminate a surface being analyzed and an optical data digitizer to sense a signal provided by the light beam reflected from the illuminated surface. The optical data digitizer is used to compare the signal received from the surface in a reference condition to subsequent signals received from the surface after surface deformation.
As is known in the art, data from the interference speckle can be used in several ways. While the specimen is stretched, the speckles translate indicating in-plane displacement and also vary in intensity indicating out-of-plane displacement. Due to the nature of materials, it can be assumed that changes from one speckle to an adjacent one are small and therefore linear. Because of this, contour maps of displacements and strains, both in-plane and out-of-plane can be constructed. The mathematical theorems and explanations of the recombination of object and reference beams are known in the art and are further described in a publication of the inventor, Optical Methods of Engineering Analysis, Cambridge University Press 1995, Gary Cloud, which is expressly incorporated herein by reference.
The speckle is itself an interference phenomenon. The formation of speckles in imaging systems can be described at any image region as the superimposition result of the coherent point spread functions for adjacent object points. The speckle created by imaging optics is referred to as a "subjective" speckle. The nature of the illuminated surface gives rise to two different classes of speckle patterns. One class is called the "fully developed" speckle pattern; it develops only from interference of light that is all polarized in the same manner. The speckle field itself will then be similarly polarized. Surfaces at which polarized light is singly scattered, such as matte finished metal, generally give rise to polarized speckle fields as do lightly scattering transmission elements such as ground glass. Matte white paint surfaces or opal glass, into which the light penetrates and is multiply scattered, depolarize the light and thus do not generate a fully developed speckle pattern. The brightness distributions of the two classes of speckle patterns differ substantially, but this difference is not important in the functioning of speckle interferometry systems.
The current invention requires the mixing of two speckle patterns, from two different scattering surfaces. When this occurs, the size of the speckles does not change appreciably, but their brightness distribution might be altered, depending on whether the patterns are mixed coherently or not (in this case coherently mixed). For the case in which the two original speckle fields are brought together coherently, the result is a third speckle pattern, differing only in detail from its two component patterns but whose size and statistical brightness distribution remain unchanged. This third speckle pattern is used in measuring motion, deformation, or strain of one of the reference surfaces.
Several different scattering surfaces are provided in the current invention. In the first, a reference surface is provided which is positioned adjacent to the surface of the specimen or surrounding the specimen. The reference surface and the specimen surface are illuminated by a beam of coherent light which has been passed through a spatial filter. The mixed speckle patterns scattered or reflected from these surfaces are recorded by the imaging system. The specimen is then subjected to a load, which causes displacement of the object's surface. This displacement causes a change in location and intensity of the various speckles in the mixed pattern. The changed patterns are again recorded by the imaging system. A computer connected to the camera captures the images and calculates displacements or strains on the object's surface.
In another embodiment of the current invention, a plate of at least partially transparent material is positioned between the laser illuminating source and the surface of the specimen. A portion of the light travels through the partially transparent reference and is scattered or reflected from the surface of the specimen. The mixed speckle patterns from the reflection off the transparent reference plate and the surface of the specimen are captured by the imaging system.
Optionally, any or all of the reference plates can be coupled to a system that translates the reference surfaces. As will be further described herein, the translation of the reference plate can be used to calculate the displacements of the surface of the specimen.
The disclosed speckle interferometry system is very good at measuring both in-plane and out-of-plane displacements. The processing of the data differs, however, depending on the displacement component sought. As the specimen is translated out of the plane of the specimen, the path lengths for the waves scattered from within a resolution element will change, causing a change of relative phase or intensity of a given speckle. As the specimen is translated in its plane, the speckle pattern is translated.
By including a capability which allows for the translation of one of the different scattering surfaces, the process known as "phase shifting interferometry" or "phase stepping interferometry" can be implemented with this invention. In this case, the scattering surface or reference plate is translated so as to determine the relative phase of a given speckle. This allows the imaging system to take brightness data for a given speckle and translate it to data that corresponds to out-of-plane displacement of the surface of the specimen. The system then uses the translation of the speckle pattern to precisely calculate the in-plane translation of the specimen surface. As such, the current system provides an efficient non-contacting system that can measure both in-plane and out-of-plane displacements and hence strains of the surface of a specimen.
As such, it is an object of this invention to provide a method and apparatus for measurement of deformations, displacements, and strains of the surface of structures of all kinds.
It is further an object of the present invention to measure the relative magnitude of displacements from an original position on different points on a surface of an object under stress.
It is yet another object of the present invention to provide an improved interferometry apparatus and technique for performing electronic speckle pattern interferometry in the analysis of motion, strain, and deformations of all kinds of structures, components, bodies and materials.
It is yet another object of the present invention to provide an interferometry apparatus which will be useful in the areas of engineering, manufacturing, medicine and natural science for making precise measurements without the necessity of heavy investment in equipment.
It is yet another object of the present invention to provide an interferometry apparatus using what is known as digital speckle pattern interferometry (DSPI) and video holography--video holographic interferometry (VHI). The apparatus is greatly simplified in comparison with traditional setups, and makes the method much more resistant to vibration and other sources of noise which tend to contaminate the results of DSPI.
It is yet another object of the present invention to provide an interferometry apparatus and method which maintains an excellent bandwidth characteristic of the traditional speckle interferometry approach.
It is yet another object of the present invention to provide an interferometry apparatus utilizing the speckle effect having a reference plate disposed before the specimen. The speckle interferometer includes a laser, a spatial filter/expander, a reference surface, a phase shifter, and recording media. Fringes occur upon making a pair of exposures of the interference patterns made before and after deformation of a rough surface. The relative magnitude of the displacements from the original position at different points on the surface can be determined from the position of the fringes. Alternatively, if phase shifting or phase stepping is used, then three or more images of the specimen are captured before and after loading, each image being taken at a different phase shift. The displacements are computed directly from the brightness data and may or may not be displayed as "fringes," displacement maps, or strain maps depending on application.
The foregoing as well as other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the appended drawings.